1. Field of the Invention
The present invention relates to thin film multilayered structures applicable to ferroelectric thin film elements using Si substrates, for example, capacitors for DRAMs and ferroelectric RAMs (FeRAM), pyroelectric elements, microactuators, thin film capacitors, small piezoelectric elements, etc., and manufacturing methods thereof. More specifically, it relates to metallic thin films epitaxially grown on Si substrates interposing buffer layers and manufacturing methods thereof.
2. Description of the Related Art
Recent years, techniques in forming thin films of dielectrics and ferroelectrics, for example, BaTiO3, SrTiO3, (Ba,Sr)TiO3 (hereafter abbreviated as BST), PbTiO3, (Pb,La)TiO3, Pb(Zr,Ti)O3 (hereafter abbreviated as PZT), (Pb,La)(Zr,Ti)O3 (hereafter abbreviated as PLZT) and Pb(Mg,Nb)O3, on Si substrates have been researched extensively.
In particular, if Pb perovskite type ferroelectrics, for example, PZT, and PLZT, having a large residual dielectric polarization could be epitaxially grown, the spontaneous polarization can be oriented in one direction, and then larger polarization values and switching characteristics can be realized. Therefore, the applicability to high density recording media is extremely increased so that requirements for establishment of methods in forming ferroelectric thin films, being excellent in crystalline properties, on Si substrates have intensified.
In the use for orienting the spontaneous polarization in one direction, the film thickness direction, as described above, a so-called MFM (metal-ferroelectric-metal) structure where a ferroelectric thin film is interposed between an upper and a lower metallic thin films (electrode layer) on a Si substrate has been generally used. However, it is difficult to improve crystalline properties of ferroelectric thin films in this structure for reasons described below, and ferroelectric thin films having fully satisfying crystalline properties have not been obtained until now.
That is, when a metallic material, for example, Al, Cu, Ag and Au, is used as a metallic thin film (lower electrode) formed on a Si substrate, a metallic oxide film is formed at the interface of the metallic thin film and the ferroelectric thin film during the formation of the ferroelectric thin film on said lower electrode. Mutual diffusion is likely to occur between the aforementioned metallic material and the Si substrate so that in the case in which a semiconductor element, etc., is formed on the Si substrate, the characteristics thereof may be changed.
A method using Pt as the metallic thin film is also taken into consideration. Pt has advantages in that it is difficult to be oxidized in air and it is likely to cause lattice matching with PZT and ferroelectrics, for example, PLZT, and BST. However, because Pt is inherently likely to form compounds with elements, for example, Si and Pb, it was feared that the characteristics of the semiconductor elements formed on the Si substrate might be changed, and compounds with ferroelectrics containing Pb formed at the interface so that the crystalline properties of ferroelectric thin films formed thereon might be degraded. A phenomenon wherein oxygen diffuses into the lower layer through grain boundaries of the Pt thin film was observed, and it was feared that, although Pt itself is difficult to be oxidized, characteristics of elements or films, for example, semiconductor elements, positioned in the under layer of the Pt thin film might be badly effected.
Regarding this point, Ir or Rh having a face-centered cubic structure has, similarly to Pt, a high conductivity, is better in processability compared to Pt, and furthermore, has a diffusion barrier function against oxygen so that the phenomenon wherein oxygen diffuses into the lower layer through the Ir thin film does not occur. Ir is not likely to react with other elements so that problems accompanied with using Pt such as the change in the characteristics of semiconductor elements and the degradation of crystalline properties of ferroelectric thin films can be suppressed.
As described above, Ir and Rh are said to be suitable materials for electrode layers to manufacture ferroelectric thin films being excellent in crystalline properties. However, regarding the forming of Ir or Rh thin film on a Si substrate, it was difficult to cause the epitaxial growth by conventional methods. For example, although Nakamura et al. formed the Ir thin film, as the electrode of the PZT thin film capacitor, on a SiO2/Si substrate by the RF magnetron sputtering method (Jpn. J. Appl. Phys. Vol.34 (1995), 5184), the obtained Ir thin film was not a epitaxial film but a film having the (111) preferred orientation. Although Horii et al. formed the Ir thin film on a YSZ/Si substrate by the sputtering method (The Japan Society of Applied Physics and Related Societies / Extended Abstracts (The 45th Meeting (1998)), only a film in which the (100) and (111) orientations are mixed with each other was obtained.
Accordingly, the object of the present invention is to provide a substrate metallic thin film which functions to form epitaxial ferroelectric thin films of excellent crystalline properties on Si substrates, and to provide a manufacturing method thereof.
The thin film multilayered structure comprises: a single crystal Si substrate; a MgO buffer layer epitaxially grown on said single crystal Si substrate; and a metallic thin film made of Ir or Rh epitaxially grown on said MgO buffer layer.
The single crystal Si substrate and the MgO buffer layer preferably fulfill the crystallographic relations:
MgO (001) // Si (001); and
MgO [100] // Si [100]
It is more preferable that said single crystal Si substrate, said MgO buffer layer and said metallic thin film fulfill the crystallographic relations:
metallic thin film (001) MgO (001) // Si (001), and
metallic thin film [100] // MgO [100] // Si [100].
The MgO buffer layer preferable has a mean surface roughness of about 1.5 nm or less, and said metallic thin film preferably has a mean surface roughness of about 1.5 nm or less.
The ferroelectric thin film element comprises, in addition to the above-explained thin film multilayered structure, a ferroelectric thin film orientationally grown on said thin film multilayered structure and an upper electrode formed on said ferroelectric thin film.
The manufacturing method of a thin film multilayered structure comprises the steps of epitaxially growing a MgO buffer layer on a single crystal Si substrate; and epitaxially growing a metallic thin film made of Ir or Rh on said MgO buffer layer.
The MgO buffer layer is preferably formed at a temperature of about 350 to 900xc2x0 C. and at a growth rate of 1.0 to 2.0 nm/min, and more preferably at a temperature of about 500 to 900xc2x0.
The manufacturing method of a ferroelectric thin film element comprises the step of, in addition to the above-explained method, orientationally growing a ferroelectric thin film is on said thin film multilayered structure and forming an upper electrode on said ferroelectric thin film.
By interposing the epitaxial MgO layer as the buffer layer on the Si substrate, and by adopting the thin film multilayered structured structure in which the metallic thin film of Ir or Rh, having a face-centered cubic structure, is formed on the MgO layer, an epitaxial metallic thin film excellent in crystalline properties and surface flatness can be formed on the Si substrate. Furthermore, functional thin films of ferroelectrics, etc., having high orientational properties with one or more axes can be formed on the epitaxial metallic thin film.
Ir and Rh are likely not to cause oxygen diffusion or react with other elements. Therefore, in the case in which the ferroelectric thin film element is formed on the Si substrate, by using the epitaxially grown Ir thin film or Rh thin film as the lower electrode of the ferroelectric thin film element, a ferroelectric thin film excellent in crystalline properties can be formed without changing the characteristics of the semiconductor element, etc., formed on the Si substrate.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.